Abstract:

To provide an X-ray CT apparatus capable of acquiring information on
energy of X-rays generated by an X-ray generating apparatus, an X-ray CT
apparatus (100) comprises: an X-ray generating apparatus for generating
X-rays and emitting the X-rays toward a subject; a first X-ray detector
having a plurality of X-ray detection channels for detecting the X-rays
emitted from the X-ray generating apparatus; a second X-ray detector for
detecting the X-rays emitted from the X-ray generating apparatus in order
to acquire information on energy of the X-rays emitted from the X-ray
generating apparatus; and an X-ray energy information identifying section
for identifying information on energy of the X-rays emitted from the
X-ray generating apparatus based on the information detected by the
second X-ray detector.

Claims:

1. An X-ray Computed Tomography (CT) apparatus comprising:an X-ray
generating apparatus configured to generate X-rays and to emit the X-rays
toward a subject;a first X-ray detector comprising a plurality of X-ray
detection channels configured to detect the X-rays emitted from said
X-ray generating apparatus;a second X-ray detector configured to detect
the X-rays emitted from said X-ray generating apparatus in order to
acquire information on energy of the X-rays emitted from said X-ray
generating apparatus; andan X-ray energy information identifying section
configured to identify information on energy of the X-rays emitted from
said X-ray generating apparatus based on the information detected by said
second X-ray detector.

2. The X-ray CT apparatus according to claim 1, wherein said second X-ray
detector is disposed at an edge of said first X-ray detector.

3. The X-ray CT apparatus according to claim 1, wherein said second X-ray
detector comprises a semiconductor detector configured to count photons,
and the information on energy is identified based on the count of
photons.

4. The X-ray CT apparatus according to claim 3, wherein said semiconductor
detector configured to count photons is made by using a material selected
from a group consisting of CdTe, CdZnTe, HgI2, PbI2 and GaAs.

5. The X-ray CT apparatus according to claim 1, wherein said second X-ray
detector comprises an X-ray detector comprising scintillators and
photodiodes and is provided thereabove with an X-ray filter, and the
information on energy is identified based on X-rays passing through said
X-ray filter.

6. The X-ray CT apparatus according to claim 1, wherein said second X-ray
detector is one of a plurality of X-ray detection channels of said first
X-ray detector.

7. The X-ray CT apparatus according to claim 1, wherein said first X-ray
detector comprises an X-ray detector comprising scintillators and
photodiodes.

8. The X-ray CT apparatus according to claim 1, wherein said second X-ray
detector comprises a plurality of X-ray detection channels.

9. The X-ray CT apparatus according to claim 8, wherein said plurality of
X-ray detection channels are arranged in a body axis direction of said
subject.

10. The X-ray CT apparatus according to claim 1, wherein said second X-ray
detector is disposed in proximity of said X-ray generating apparatus.

11. The X-ray CT apparatus according to claim 10, wherein said second
X-ray detector comprises a semiconductor detector configured to count
photons, and the information on energy is identified based on the count
of photons.

12. The X-ray CT apparatus according to claim 11, wherein said
semiconductor detector configured to count photons is made by using a
material selected from a group consisting of CdTe, CdZnTe, HgI2,
PbI2 and GaAs.

13. The X-ray CT apparatus according to claim 10, wherein said second
X-ray detector comprises an X-ray detector comprising scintillators and
photodiodes and is provided thereabove with an X-ray filter, and the
information on energy is identified based on X-rays passing through said
X-ray filter.

14. The X-ray CT apparatus according to claim 1, wherein said X-ray
generating apparatus is configured to generate X-rays with a plurality of
energy spectra based on a plurality of X-ray tube voltages.

15. The X-ray CT apparatus according to claim 14, wherein said X-ray
generating apparatus is configured to generate the X-rays with a
plurality of energy spectra by a single X-ray generating apparatus
switching between a plurality of X-ray tube voltages on a view-by-view or
views-by-views basis.

16. The X-ray CT apparatus according to claim 1, further comprising: a
data correcting section configured to correct data based on X-rays
detected by said first X-ray detector, the correction made based on the
information on energy identified by said X-ray energy information
identifying section.

17. The X-ray CT apparatus according to claim 1, further comprising: a
display section configured to display the information on energy of the
X-rays emitted from said X-ray generating apparatus.

18. An X-ray Computed Tomography (CT) apparatus comprising:an X-ray
generating apparatus configured to generate X-rays and to emit the X-rays
toward a subject;an X-ray filter configured to form an X-ray beam from
the X-rays emitted by said X-ray generating apparatus;a first X-ray
detector comprising a plurality of X-ray detection channels configured to
detect the X-rays within the X-ray beam;a second X-ray detector
configured to detect the X-rays within the X-ray beam in order to acquire
information on energy of the X-rays;an X-ray energy information
identifying section configured to identify information on energy of the
X-rays within the X-ray beam based on the information detected by said
second X-ray detector; andan image reconstruction section configured to
generate a tomographic image based on the information on energy
identified by said X-ray energy information identifying section.

19. The X-ray CT apparatus according to claim 18, wherein said second
X-ray energy detector is positioned with respect to one of said X-ray
filter and said first X-ray detector.

20. An X-ray Computed Tomography (CT) method comprising:generating X-rays
and emitting the X-rays toward a subject using an X-ray generating
apparatus;detecting the X-rays emitted by the X-ray generating apparatus
using a first X-ray detector, the first X-ray detector including a
plurality of X-ray detection channels;detecting the X-rays emitted by the
X-ray generating apparatus using a second X-ray detector in order to
acquire information on energy of the X-rays emitted by the X-ray
generating apparatus;identifying information on energy of the X-rays
emitted by the X-ray generating apparatus using an X-ray energy
information identifying section, the information on energy identified
based on the information detected by the second X-ray detector;
andgenerating a tomographic image using an image reconstruction section
based on the information on energy identified by the X-ray energy
information identifying section.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001]This application claims the benefit of Japanese Patent Application
No. 2008-049433 filed Feb. 29, 2008, which is hereby incorporated by
reference in its entirety.

[0003]In general, an X-ray CT apparatus generates X-rays with a desired
energy spectrum by controlling an X-ray tube voltage of an X-ray tube.
There is also a known technique for obtaining a tomographic image based
on CT values obtained using X-rays with an energy spectrum generated from
an X-ray tube using a higher X-ray tube voltage and those obtained using
X-rays with an energy spectrum generated from the X-ray tube using a
lower X-ray tube voltage, as disclosed in Japanese Patent Application
Laid Open No. 2004-065975, for example. More particularly, the technique
includes a technique of obtaining a weighted subtraction image in which a
certain material within a subject is enhanced using the fact that X-ray
absorption coefficients of materials within the subject are different
from one another in response to different energy spectra of X-rays. A
technique of obtaining a CT image using projection data based on X-rays
with two different energy spectra is sometimes referred to as dual energy
imaging.

BRIEF DESCRIPTION OF THE INVENTION

[0004]Conventionally, energy of X-rays generated from an X-ray tube is
identified by a specified X-ray tube voltage, and energy of actually
generated X-rays cannot be accurately known. In recent years, it is
desirable to accurately know energy of X-rays actually emitted by an
X-ray tube in dual energy imaging as described above that rely upon the
energy of X-rays.

[0005]Accordingly, embodiments of the present invention provide an X-ray
CT apparatus capable of acquiring information on energy of X-rays
generated by an X-ray generating apparatus.

[0006]An X-ray CT apparatus in a first aspect includes an X-ray generating
apparatus for generating X-rays and emitting the X-rays toward a subject;
a first X-ray detector having a plurality of X-ray detection channels for
detecting the X-rays emitted from said X-ray generating apparatus; a
second X-ray detector for detecting the X-rays emitted from said X-ray
generating apparatus in order to acquire information on energy of the
X-rays emitted from said X-ray generating apparatus; and an X-ray energy
information identifying section for identifying information on energy of
the X-rays emitted from said X-ray generating apparatus based on the
information detected by said second X-ray detector.

[0007]An X-ray CT apparatus in a second aspect of the invention is the
X-ray CT apparatus of the first aspect, wherein said second X-ray
detector is disposed at an edge of said first X-ray detector.

[0008]An X-ray CT apparatus in a third aspect of the invention is the
X-ray CT apparatus of the first or second aspect, wherein said second
X-ray detector is a semiconductor detector capable of counting photons,
and said information on energy is identified based on said count of
photons.

[0009]An X-ray CT apparatus in a fourth aspect of the invention is the
X-ray CT apparatus of the third aspect, wherein said semiconductor
detector capable of counting photons is made by using a material selected
from a group consisting of CdTe, CdZnTe, HgI2, PbI2 and GaAs.

[0010]An X-ray CT apparatus in a fifth aspect of the invention is the
X-ray CT apparatus of the first or second aspect, wherein said second
X-ray detector is an X-ray detector comprising scintillators and
photodiodes and is provided thereabove with an X-ray filter, and said
information on energy is identified based on X-rays passing through said
X-ray filter.

[0011]An X-ray CT apparatus in a sixth aspect of the invention is the
X-ray CT apparatus of any one of the first through fifth aspects, wherein
said second X-ray detector is part of a plurality of X-ray detection
channels of said first X-ray detector.

[0012]An X-ray CT apparatus in a seventh aspect of the invention is the
X-ray CT apparatus of any one of the first through fifth aspects, wherein
said first X-ray detector is an X-ray detector comprising scintillators
and photodiodes.

[0013]An X-ray CT apparatus in an eighth aspect of the invention is the
X-ray CT apparatus of any one of the first through seventh aspects,
wherein said second X-ray detector comprises a plurality of X-ray
detection channels.

[0014]An X-ray CT apparatus in a ninth aspect of the invention is the
X-ray CT apparatus of the eighth aspect, wherein said plurality of X-ray
detection channels are arranged in a body axis direction of said subject.

[0015]An X-ray CT apparatus in a tenth aspect of the invention is the
X-ray CT apparatus of the first aspect, wherein said second X-ray
detector is disposed in proximity of said X-ray generating apparatus.

[0016]An X-ray CT apparatus in an eleventh aspect of the invention is the
X-ray CT apparatus of the tenth aspect, wherein said second X-ray
detector is a semiconductor detector capable of counting photons, and
said information on energy is identified based on said count of photons.

[0017]An X-ray CT apparatus in a twelfth aspect of the invention is the
X-ray CT apparatus of the eleventh aspect, wherein said semiconductor
detector capable of counting photons is made by using a material selected
from a group consisting of CdTe, CdZnTe, HgI2, PbI2 and GaAs.

[0018]An X-ray CT apparatus in a thirteenth aspect of the invention is the
X-ray CT apparatus of the tenth aspect, wherein said second X-ray
detector is an X-ray detector comprising scintillators and photodiodes
and is provided thereabove with an X-ray filter, and said information on
energy is identified based on X-rays passing through said X-ray filter.

[0019]An X-ray CT apparatus in a fourteenth aspect of the invention is the
X-ray CT apparatus of any one of the first through thirteenth aspects,
wherein said X-ray generating apparatus generates X-rays with a plurality
of energy spectra based on a plurality of X-ray tube voltages.

[0020]An X-ray CT apparatus in a fifteenth aspect of the invention is the
X-ray CT apparatus of the fourteenth aspect, wherein said X-ray
generating apparatus generates said X-rays with a plurality of energy
spectra by a single X-ray generating apparatus switching between a
plurality of X-ray tube voltages on a view-by-view or views-by-views
basis.

[0021]An X-ray CT apparatus in a sixteenth aspect of the invention is the
X-ray CT apparatus of any one of the first through fifteenth aspects,
further comprising: a data correcting section for correcting data based
on X-rays detected by said first X-ray detector, the correction being
made based on the information on energy identified by said X-ray energy
information identifying section.

[0022]An X-ray CT apparatus in a seventeenth aspect of the invention is
the X-ray CT apparatus of any one of the first through sixteenth aspects,
further comprising: a display section for displaying the information on
energy of the X-rays emitted from said X-ray generating apparatus.

[0023]According to the embodiments described herein, an X-ray CT apparatus
is provided that is capable of acquiring information on energy of X-rays
generated by an X-ray generating apparatus by making the X-ray CT
apparatus comprise a second X-ray detector for detecting X-rays emitted
from the X-ray generating apparatus and have an X-ray energy information
acquiring section for acquiring information on energy of X-rays detected
by the second X-ray detector.

[0025]FIG. 2 is a diagram for explaining an exemplary second X-ray
detector that may be used with the X-ray CT apparatus shown in FIG. 1.

[0026]FIG. 3 is a diagram for explaining another exemplary second X-ray
detector that may be used with the X-ray CT apparatus shown in FIG. 1.

[0027]FIGS. 4A, 4B, and 4C are diagrams for explaining identification of
energy information.

[0028]FIG. 5 is a diagram for explaining use of a second X-ray detector
with the X-ray CT apparatus shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0029]FIG. 1 is a block diagram of a configuration of an exemplary X-ray
CT apparatus 100. The X-ray CT apparatus 100 comprises an operator
console 1, an imaging table 10, and a scan gantry 20.

[0030]The operator console 1 comprises an input device 2 for accepting an
input from a human operator, such as a keyboard or a mouse, a central
processing apparatus 3 for executing pre-processing, image reconstruction
processing, post-processing, etc., a data collection buffer 5 for
collecting X-ray detector data collected at the scan gantry 20. The
operator console 1 also comprises a monitor 6 for displaying a
tomographic image image-reconstructed from projection data obtained by
pre-processing the X-ray detector data, and a storage device 7 for
storing programs, the X-ray detector data, projection data, and X-ray
tomographic images. Imaging conditions are input via the input device 2
and stored in the storage device 7. The imaging table 10 comprises a
cradle 12 for laying thereon a subject HB to be carried into/out of a
bore of the scan gantry 20. The cradle 12 is vertically and horizontally
moved/translated by a motor incorporated in the imaging table 10.

[0032]The multi-row X-ray detector 24 is provided with a first X-ray
detector 241 having a plurality of X-ray detection channels for detecting
X-ray projection data of the subject and a second X-ray detector 242 for
acquiring information on energy of emitted X-rays.

[0033]The scan gantry 20 further comprises a rotation control section 26
for controlling rotation of the rotating section 15 having the X-ray tube
21 that rotates around a body axis of the subject HB, and a gantry
control section 29 for communicating control signals or the like with the
operator console 1 and imaging table 10. The beamforming X-ray filter 28
is provided for reducing subject exposure to radiation, and it is an
X-ray filter configured to have a thickness that is smallest in a
direction of X-rays traveling toward a center of rotation, which is an
imaging center, and becomes larger toward peripheries to absorb more
X-rays.

[0037]The beam hardening correction processing executes processing of beam
hardening correction on projection data. Beam hardening refers to a
phenomenon that an X-ray energy distribution of continuous X-rays varies
as such X-rays travel inside a material, resulting in variation in CT
values (brightness) in a cross section. The beam hardening correction is
applied to projection data in a slice direction and a channel direction.

[0038]The image reconstruction processing involves receiving pre-processed
projection data and reconstructing an image based on the projection data.
The projection data is subjected to fast Fourier transform (FFT) for
transforming the data into that in a frequency domain, and the resulting
data is convolved with a reconstruction function Kernel(j) in a spatial
domain and subjected to inverse Fourier transformation. The image
reconstructing section 34 then applies three-dimensional backprojection
processing to the projection data convolved with the reconstruction
function Kernel(j), and thereafter, applies post-processing to the data
to convert it into a tomographic image having CT values on a
pixel-by-pixel basis, which tomographic image is obtained in each cross
section (xy plane) along a body axis direction of the subject HB (Z-axis
direction). The image reconstructing section 34 stores these tomographic
images in the storage device 7.

[0039]The dual energy image reconstruction processing involves
image-reconstructing a two-dimensional distribution tomographic image of
X-ray tube voltage dependent information relating to a distribution of a
certain material (atoms), which image is sometimes called a tomographic
image by dual energy imaging, from X-ray projection data obtained using a
lower X-ray tube voltage kV1 and that obtained using a higher X-ray tube
voltage kV2. The tomographic images in dual energy imaging that can be
obtained include so-called water-equivalent image, fat-equivalent image,
contrast-equivalent image, bone-equivalent image or the like, in which
water, fat, contrast agent, bone or the like is substantially eliminated,
respectively.

[0040]The X-ray energy information identifying section 34 uses data based
on a result detected by the second X-ray detector 242 to calculate or
estimate energy information and identify it.

[0041]The data correcting section 35 corrects data based on X-rays
detected by the first X-ray detector 241 based on the energy information
identified by the X-ray energy information identifying section 34.

[0042]Next, acquisition of X-ray energy information by the second X-ray
detector 242 and identification of X-ray energy by the X-ray energy
information identifying section 34 using the aforementioned X-ray CT
apparatus 100 will be described in detail with reference to embodiments.

[0043]Embodiment 1 shows an example employing a so-called photon-counting
X-ray detector, which is a semiconductor detector capable of counting
photons, as the second X-ray detector 242.

[0044]FIG. 2 is a diagram showing a disposition of a second X-ray detector
according to the present embodiment. In the drawing, the second X-ray
detector 242 constructed from a photon-counting X-ray detector in which a
material directly reactive with X-ray photons such as a CdTe
semiconductor is employed as a detector element material is disposed
adjacent to an edge of the first X-ray detector 241 in a rotation
direction (channel direction), which detector 241 is a so-called
scintillating X-ray detector constructed from a combination of
scintillators and photodiodes. In the present embodiment, it is assumed
that the second X-ray detector 242 has eight X-ray detection channels
(kVRef1-kVRef8). Moreover, the second X-ray detector 242 may be disposed
at any other position than the edge of the first X-ray detector 241 in
the rotation direction (channel direction), insofar as its channels lie
at a position unlikely to be obstructed by the subject. In this case,
X-rays having a similar solid angle and similar quality to those of the
first X-ray detector for detecting X-rays in the imaging region can be
captured by the second X-ray detector 242. Specifically, since the
emitted X-rays equally pass through the X-ray filter near the X-ray tube
21 and beamforming X-ray filter 28, thereby forming a cone beam CB, they
have similar X-ray quality for the detectors 241 and 242. Moreover, since
the length from the X-ray focal spot is insignificantly different in
geometry, the X-rays have a similar signal level for the detectors 241
and 242.

[0045]As X-rays emitted using the X-ray CT apparatus 100 impinge upon the
first X-ray detector 241 for each view, they also impinge upon the
channels kVRef1-kVRef8 of the second X-ray detector 242, and a signal is
counted for each channel based on an electrical charge generated at a
semiconductor element depending upon energy of X-ray photons. At that
time, a threshold(s) for energy may be defined to count separately for
energy ranges. From a result of the count, the X-ray energy information
identifying section 34 determines energy information at those channels
based on a calculation method defined as appropriate for
likely-to-be-optimal energy (for example, an energy range containing a
peak of the count, or an average of a plurality of energy ranges
containing a peak of the count), and identifies energy information in a
current view based on a calculation method defined as appropriate for
optimal energy information based on the energy information for the eight
channels (for example, an average of eight pieces of energy information).

[0046]For example, when X-rays are emitted while switching the X-ray tube
voltage between 80 kV and 140 kV on a view-by-view or views-by-views
basis in dual energy imaging, thresholds that divide photon energy into
three ranges, 80 kV, 140 kV and middle kV, are provided, and energy
information representing 80 kV, 140 kV or middle kV is acquired for every
view(s). As a result, one can know whether X-ray projection data is under
80 kV, 140 kV, or transient kV during switching of the X-ray tube
voltage, for each view.

[0047]It should be noted that in the aforementioned embodiment, a
semiconductor material employed in the photon-counting X-ray detector is
not limited to CdTe, and CdZnTe, HgI2, PbI2, GaAs, etc. may be
employed.

[0048]Moreover, while the present invention is achieved in the embodiment
above by employing a most widespread scintillating X-ray detector for the
first X-ray detector and adding the second X-ray detector to the
conventional multi-row X-ray detector, the photon-counting X-ray detector
may be employed for the first X-ray detector, that is, partial channels
in the photon-counting X-ray detector may be served as the second X-ray
detector.

[0049]Furthermore, some scintillating X-ray detectors have X-ray dose
correction channel(s), in which case the second X-ray detector of the
present embodiment may be disposed adjacent to such X-ray dose correction
channel(s).

[0050]Additionally, while energy information with higher accuracy may be
obtained by employing more X-ray detector elements for the second X-ray
detector in the embodiment above, the X-ray detector elements are not
limited to provision in a plural number, and one or more X-ray detector
elements may be provided.

[0051]Embodiment 2 shows an example in which outermost channels of the
multi-row X-ray detector 24 in the rotation direction is used for the
second X-ray detector 242. That is, the second X-ray detector is
constructed from a scintillating X-ray detector in Embodiment 2.

[0052]FIG. 3 is a diagram showing a disposition of a second X-ray detector
according to the present embodiment. In the drawing, there is shown an
example in which eight energy acquisition channels (kVRef1-kVRef8)
constituting the second X-ray detector 242 are provided. It should be
noted that in the present embodiment, similarly to Embodiment 1, the
second X-ray detector 242 may not necessarily be outermost channels of
the multi-row X-ray detector 24 in the rotation direction (channel
direction), insofar as its channels lie at a position unlikely to be
obstructed by the subject. Moreover, in the present embodiment, similarly
to Embodiment 1, X-rays similar to those at the first X-ray detector can
be captured at the second X-ray detector 242.

[0053]Furthermore, some scintillating X-ray detectors have X-ray dose
correcting channel(s), in which case the second X-ray detector may be
disposed adjacent to such X-ray dose detecting channel(s) and/or it may
be served as the X-ray dose detecting channel(s) as well. Moreover, the
second detector may be disposed in proximity of or adjacent to the
multi-row X-ray detector 24 as a detector separate from the multi-row
X-ray detector 24, rather than as the outermost channels of the multi-row
X-ray detector 24 in the rotation direction.

[0054]Furthermore, as shown in FIG. 3, the second X-ray detector 242 is
provided above its energy acquisition channels with an X-ray filter 243
having a varying thickness. The X-ray filter is assumed to have
thicknesses t(1)-t(8). The X-ray filter 243 having a varying thickness
causes X-rays with different energy wavelength bands to be input to the
energy acquisition channels kVRef1-kVRef8.

[0055]FIG. 4A shows outputs DkVRef(1)-DkVRef(8) of the energy acquisition
channels kVRef1-kVRef8 when X-rays based on an X-ray tube voltage are
input to the energy acquisition channels kVRef1-kVRef8 shown in FIG. 4B.
FIG. 4C shows outputs DCkVRef(1)-DCkVRef(8) after the outputs
DkVRef(1)-DkVRef(8) are corrected for attenuation by the thickness of the
X-ray filter 243. The outputs DCkVRef(1)-DCkVRef(8) after attenuation by
the thickness of the X-ray filter is corrected can be determined
according to EQ. 1 below, where an output of an energy acquisition
channel is represented as DkVRef(i), i=1-8, the thickness of the X-ray
filter 242 is represented as t(i), an X-ray linear absorption coefficient
of the material of the X-ray filter 242 is represented as p, and an
output after X-ray filter thickness correction is represented as
DCkVRef(i):

DCkVRef(i)=DkVRef(i)e.sup.ρt(i) EQ. 1

[0056]Although the output after the X-ray filter thickness correction is
ideally equivalent to that of X-rays not passing through the filter, it
exhibits variation due to energy of X-rays in practice. In the present
embodiment, the variation in the output due to a difference in energy is
actively used. Specifically, the X-ray energy information identifying
section 34 stores therein a reference output after X-ray filter thickness
correction (a solid dotted line in the graph of FIG. 4C for each level of
energy beforehand, compares actual output energy with the reference, and
identifies the measured output energy as information on energy of X-rays
by calculation or estimation. For example, an average of the measured
output energy may be served as energy information for a current view by
calculation or estimation on a channel-by-channel basis.

[0057]Moreover, while energy information with higher accuracy may be
obtained by employing more X-ray detector elements for the second X-ray
detector in the embodiment above, the X-ray detector elements are not
limited to provision in a plural number, and one or more X-ray detector
elements may be provided.

[0058]Next, an example in which a second X-ray detector similar to that in
Embodiment 1 or 2 is disposed in proximity of the X-ray tube 21 will be
described as a variation.

[0059]FIG. 5 is a diagram showing a disposition of a second X-ray
detector. In FIG. 5, a second X-ray detector 441 is disposed in proximity
of the X-ray tube 21. Such a second X-ray detector 441 is not obstructed
by a subject even if the subject is large-sized. In this case, X-rays
input to the second X-ray detector 441 and those input to the multi-row
X-ray detector 24 through the beamforming filter have different quality,
and therefore, it is preferable to dispose an X-ray filter equivalent to
the beamforming filter above the second X-ray detector, in addition to an
X-ray filter similar to the aforementioned X-ray filter 242. It should be
noted that acquisition of information on energy of X-rays may be achieved
in this variation in a similar manner to that in Embodiment 1 or 2.

[0060]It should be noted that the information on energy of X-rays acquired
in all embodiments above may be appended to X-ray projection data on, for
example, a view-by-view basis.

[0061]Moreover, the information on energy of X-rays acquired in all
embodiments above may be continuously displayed on the monitor 6 during a
scan for monitoring the energy information.

[0062]Furthermore, the information on energy of X-rays acquired in all
embodiments above may be used in data correction applied in the data
correcting section 35 for obtaining a tomographic image. For example,
when correction on X-ray projection data, such as beam hardening
correction, executed at the image processing section 33 employs a
function of the X-ray tube voltage (energy of X-rays), the correction may
be performed using the acquired energy information based on the second
X-ray detector in such a function. Alternatively, in a case that after
correction using a function with a predefined X-ray tube voltage (energy
of X-rays) has been performed, the X-ray energy information used in the
correction is different from the energy information acquired based on the
second X-ray detector, the X-ray projection data after the correction may
be additionally subjected to correction that provides data as if the
energy information acquired based on the second X-ray detector were used
in the correction.

[0063]Moreover, another example of data correction may involve converting
data of a subject acquired using X-rays with a certain energy spectrum
into data acquired using X-rays with a different energy spectrum (which
will be referred to as X-ray energy correction hereinbelow). In
particular, one example of such correction may involve converting
projection data under an X-ray tube voltage of 80 kV and that under an
X-ray tube voltage of 140 kV acquired by dual energy imaging into
projection data corresponding to an X-ray tube voltage of 120 kV.

[0064]It can be considered that the X-ray energy correction may be
employed in optimizing noise by changing the X-ray tube voltage kV
according to variation in composition or shape of the subject, for
example, in a view direction or in a body axis direction, along with or
instead of a change in X-ray tube current known in the art.

[0065]The X-ray energy correction may be achieved by several methods. In
one embodiment, a scout scan is performed using the dual energy imaging
technique to acquire information on composition of the subject. An X-ray
absorption coefficient that depends upon composition and energy is
applied to an image obtained by imaging using an arbitrary X-ray tube
voltage, and the image is converted into data corresponding to a desired
X-ray tube voltage for each component of the subject. In another
embodiment, a weighted subtraction image obtained by performing dual
energy imaging is converted for each component of the subject into data
corresponding to a desired X-ray tube voltage based on an X-ray
absorption coefficient that depends upon composition and energy.

[0066]Then, by applying the energy information identified as described
above to such X-ray energy correction, accurate X-ray energy correction
can be achieved.

[0067]It should be noted that the dual energy imaging technique in the
embodiments above may be achieved by several methods. In one embodiment,
a method includes switching the X-ray tube voltage of a single X-ray tube
on a rotation-by-rotation basis to generate X-rays with the plurality of
energy distributions as described earlier. In another embodiment, a
method includes switching the X-ray tube voltage of a single X-ray tube
on a view-by-view or views-by-views basis to generate X-rays with the
plurality of energy distribution as described earlier. In yet another
embodiment, a method includes using a plurality of X-ray tubes having
different X-ray tube voltages to generate X-rays with the plurality of
energy distributions as described earlier.